1. Field of the Invention
The present invention relates to a reference position detecting device for generating a reference signal for every movement or rotation by a predetermined amount of a moving object for use in an apparatus for measuring the moving status of a moving object.
The present invention also relates to an encoder for photoelectrically measuring the moving or rotating state of an object, and more particularly to an encoder in which a light beam, particularly a coherent light beam is introduced into a diffraction grating affixed on the object to form interference fringes by the interference of the diffracted lights from said diffraction grating, and a reference position signal for measuring the moving or rotating state of the object can be effectively and precisely obtained by counting the number of interference fringes.
2. Related Background Art
Recent precision equipment such as numerically controlled machine tools and steppers for semiconductor manufacture require precise position measurement of submicron order.
For such submicron measurement there is already known a linear encoder utilizing interference fringes formed by the diffracted lights from a moving object irradiated with a coherent light such as a laser beam.
Such a linear encoder is already disclosed for example in the U.S. Pat. Nos. 3,726,595 and 3,738,753, Japanese Utility Model Laid-Open 81510/1982, Japanese Patents Laid-Open 207805/1982, 19202/1982, 19203/1982 and 98302/1985.
On the other hand, a photoelectric rotary encoder has a so-called index scale structure as shown in FIG. 1, provided with a main scale 31 composed of opaque portions and translucent portions of the same pitch formed along the periphery of a disk 35 connected to a rotary shaft 30, a fixed index scale 32 composed of opaque portions and translucent portions of the same pitch as those of said main scale, and light emitting means 33 and light receiving means 34 positioned across said both index scales. In this structure the rotation of the main scale generates a signal synchronized with the pitch of the opaque and translucent portions of both scales, and said signal is subjected to frequency analysis to detect the variation in the rotary speed of the rotary shaft.
To obtain a reference position signal, the disk 35 is provided with a slit pattern 36, which functions in combination with a light source 38 and light receiving means 37.
Thus an output pulse signal is obtained for every rotation of the disk 35, and is utilized for checking the measurement error and conducting absolute measurements.
In this method, however, the light receiving means 37 receives the light only when the light beam passes through the pattern 36, and the reference position is defined when the output signal from the light receiving means 37 exceeds a predetermined level. Consequently, the reference position varies according to the moving direction of the pattern 36, and is also affected by the intensity of the light beam and the sensitivity of the light receiving means.
It is also possible to irradiate two overlapping slit arrays with incoherent light such as from a light-emitting diode and to define the reference position when the transmitted light reaches maximum. The resolving power in this method is defined by the maximum spatial frequency (minimum pitch) of the slit array, and can only be improved by employing a smaller pitch of the slits. However, the use of a smaller pitch, for example close to the wavelength of the light source, reduces the transmitted light due to diffraction, and requires a precise adjustment of the distance of two slit arrays.